Human Physiology, 14th edition (2016)

(Tina Sui) #1

244


9.1 NEURAL CONTROL OF


INVOLUNTARY EFFECTORS


The autonomic nervous system helps regulate the activi-


ties of cardiac muscle, smooth muscles, and glands. In this


regulation, impulses are conducted from the CNS by an


axon that synapses with a second autonomic neuron. It is


the axon of this second neuron in the pathway that inner-


vates the involuntary effectors.


Autonomic motor nerves innervate organs whose func-
tions are not usually under voluntary control. The effectors that
respond to autonomic regulation include cardiac muscle (the
heart), smooth muscles, and glands. These effectors are part
of the visceral organs (organs within the body cavities) and of
blood vessels. The involuntary effects of autonomic innerva-
tion contrast with the voluntary control of skeletal muscles by
way of somatic motor neurons.

Autonomic Neurons

Neurons of the peripheral nervous system (PNS) that conduct
impulses away from the central nervous system (CNS) are
known as motor, or efferent, neurons (chapter 7, section 7.1).
There are two major categories of motor neurons: somatic
and autonomic. Somatic motor neurons have their cell bodies
within the CNS and send axons to skeletal muscles, which are
usually under voluntary control. This was briefly described in
chapter 8 (see fig. 8.28), in the section on the reflex arc. The
control of skeletal muscles by somatic motor neurons is dis-
cussed in depth in chapter 12, section 12.5.
Unlike somatic motor neurons, which conduct impulses along
a single axon from the spinal cord to the neuromuscular junction,
autonomic motor control involves two neurons in the efferent path-
way ( fig. 9.1 and table 9.1 ). The first of these neurons has its cell
body in the gray matter of the brain or spinal cord. The axon of this
neuron does not directly innervate the effector organ but instead
synapses with a second neuron within an autonomic ganglion (a
ganglion is a collection of cell bodies outside the CNS). The first
neuron is thus called a preganglionic neuron. The second neuron
in this pathway, called a postganglionic neuron, has an axon that
extends from the autonomic ganglion to an effector organ, where it
synapses with its target tissue ( fig. 9.1 ).
Preganglionic autonomic fibers originate in the midbrain
and hindbrain and in the upper thoracic to the fourth sacral levels
of the spinal cord. Autonomic ganglia are located in the head,
neck, and abdomen; chains of autonomic ganglia also parallel
the right and left sides of the spinal cord. The origin of the pre-
ganglionic fibers and the location of the autonomic ganglia help
to distinguish the sympathetic and parasympathetic divisions of
the autonomic system, discussed in later sections of this chapter.

Sofia was nervous about her upcoming exam, and to
quiet her anxiety she placed a finger on her radial artery
and concentrated as she attempted to lower her pulse
rate. It felt good to close her eyes! Her pupils had been
dilated for an eye exam that morning. She sniffed and
took a cold pill from her purse. It contained pseudo-
ephedrine, with a warning that people with hypertension
should be careful. Well, her blood pressure was under
control with a combination of drugs that included aten-
olol. When the exam arrived, she took a puff from her
asthma inhaler and began.
Some of the new terms and concepts you will
encounter include:


  • Alpha and beta-adrenergic receptor types and their
    effects

  • Atropine, atenolol, pseudoephedrine, albuterol, and
    sympathomimetics


Clinical Investigation


Figure 9.1 The autonomic system
has preganglionic and post-ganglionic
neurons. The preganglionic neurons of the
autonomic system have cell bodies in the
CNS, whereas the postganglionic neurons
have cell bodies within autonomic ganglia.
The sympathetic and parasympathetic
divisions differ in the particular locations of
their preganglionic neuron cell bodies within
the CNS, and in the location of their ganglia.


CNS

Autonomic
ganglion Involuntary
effector

Smooth
muscle

Postganglionic
neuron

Preganglionic
neuron

LEARNING OUTCOMES


After studying this section, you should be able to:


  1. Describe the organization of autonomic motor
    neurons.

  2. Describe how neural regulation of smooth and
    cardiac muscles differs from neural regulation of
    skeletal muscles.

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